1. Field of the Invention
The present invention is related to a method for producing a compound semiconductor light-emitting device and particularly to a production method that can improve at least one of transparency, sheet resistance, planar distribution of sheet resistance, and contact resistance regarding a transparent conductive oxide film included in a compound semiconductor light-emitting device.
2. Description of the Background Art
Compound semiconductor light-emitting devices that can emit the three primary color lights of red, green and blue are indispensable in order to utilize the light-emitting devices for various illumination uses. Regarding light-emitting diodes (LEDs), it has not been possible until recent years to utilize LEDs for various illumination uses because the blue LED among LEDs of the three primary colors has not been well-completed and not been available.
However, after the blue LED formed with nitride semiconductor has been developed in the 1990s, illumination products including LEDs are utilized not only for traffic signals but also for backlights in liquid crystal monitors, backlights in liquid crystal televisions and further various illumination uses at home.
Recently, liquid crystal televisions equipped with LED backlights begin to become widely used in a rapid pace in association with their price decline. In addition, illumination devices using LEDs have merits of enabling lower power consumption, smaller space occupied by them, and free of mercury preferably to the environment, as compared with the conventional illumination devices. After the summer of 2009, illumination devices using LEDs have been put on the market at much less prices as compared with those before and thus become popular in a very rapid pace.
In the meantime, light emitted from an illumination device, a backlight of a liquid crystal television, or the like should necessarily be white light. In general, white light obtainable using an LED can be realized by a combination of a blue LED and a yellow YAG (yttrium-aluminum-garnet) phosphor or a combination of a blue LED, a green phosphor and a red phosphor. In other words, a blue LED is needed in the case of obtaining white light utilizing an LED. For this reason, it is desired to provide a method that can produce bright blue LEDs in large amounts at low prices.
In general, III-V compound semiconductors containing nitrogen as a V-group element, such as gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN) and mixed crystals thereof are used for light-emitting layers included in LEDs and laser diodes (LDs) that can emit lights of shorter wavelengths such as blue and bluish green lights.
FIG. 6 shows a blue LED of a well-known double hetero-junction type in a schematic cross-sectional view. In production of this blue LED, an Si-doped n-type GaN lower clad layer 102, an InGaN light-emitting layer 103, a Mg-doped p-type AlGaN upper clad layer 104, and a contact layer 105 are deposited in this order on a sapphire substrate 101. Deposited on contact layer 105 is a transparent conductive oxide film 108 on a partial area of which a p-side electrode 106 is provided. On the other hand, lower clad layer 102 is partly exposed by etching and then an n-side electrode 107 is provided on the exposed area.
When electric current is injected through p-side electrode 106 in the LED of FIG. 6, the current is dispersed in the planar direction of conductive oxide film 108. Then, the dispersed current is injected through contact layer 105 and upper clad layer 104 into a broad area of light-emitting layer 103 thereby causing light emission from the broad area of light-emitting layer 103.
Light emitted upward from light-emitting layer 103 is transmitted through upper clad layer 104, contact layer 105 and transparent conductive oxide film 108 and then taken out to the outside. By using a highly transparent material such as ITO (indium tin oxide) for transparent conductive oxide film 108, it becomes possible to reduce light loss when light emitted from light-emitting layer 103 is transmitted through transparent conductive oxide film 108. Further, since conductive oxide film 108 such as of ITO has lower resistance as compared with contact layer 105, diffusion of the injected current is enhanced to spread widely so as to increase light-emitting area of light-emitting layer 103 thereby improving the light-emitting efficiency of the device.
On the other hand, while conductive oxide film 108 has lower resistance as compared with contact layer 105, it exhibits a relatively high sheet resistance value in a range of 20Ω/□ to 60Ω/□. Further, the conductive oxide film tends to include relatively higher and lower sheet resistance areas in a mixed state depending on its partial areas. Therefore, the compound semiconductor light-emitting device including the transparent conductive oxide film has problems in which an operation voltage Vf thereof tends to become higher and light emission therefrom tends to become non-uniform depending on partial areas of the light-emitting layer.
In order to improve these problems, it is ideal to reduce the sheet resistance value of transparent conductive oxide film 108 to 20Ω/□ or less and preferably to 10Ω/□ or less. For the purpose of reducing the sheet resistance of the conductive oxide film, it is conceivably possible to improve the crystallinity of the conductive oxide film by annealing so as to reduce the sheet resistance of the conductive oxide film. However, there is a problem that a bonding state at an interface between conductive oxide film 108 and contact layer 105 is changed by the annealing to impair a stable bonding state such as of Ga—O, N—O, compounds of H, etc. at the interface, leading to higher contact resistance.
For the purpose of reducing the sheet resistance of the conductive oxide film, it is also conceivably possible to increase oxygen defect density in the crystal thereby increasing charge carrier density. However, the work function of the conductive oxide film is decreased as the carrier density is increased, whereby the electron potential is increased on the conductive oxide film side at the interface between conductive oxide film 108 and contact layer 105. For this reason, holes are poorly injected from the conductive oxide film to the contact layer, thereby causing a problem of increasing the contact resistance between conductive oxide film 108 and contact layer 105.
Under the circumstances, in the light-emitting device disclosed in Japanese Patent Laying-Open No. 2007-287786, it is attempted to reduce the sheet resistance of the transparent conductive oxide film with maintaining low contact resistance between the conductive oxide film and the contact layer by carrying out two-stage annealing including first annealing and second annealing. In the first annealing of Japanese Patent Laying-Open No. 2007-287786, the annealing is carried out at a temperature in a range of 250° C. to 600° C. in an atmosphere containing oxygen so as to reduce the contact resistance between the transparent conductive oxide film and the contact layer. Subsequently, in the second annealing, the annealing is carried out at a temperature in a range of 200° C. to 500° C. in an atmosphere free of oxygen (e.g., a N2 gas atmosphere) so as to reduce the sheet resistance of the transparent conductive oxide film.
It is conventionally usual that an oxide film such as of ITO or IZO (indium zinc oxide) used for the transparent conductive oxide film is annealed in a N2 gas atmosphere at the same pressure as the atmospheric pressure and then is cooled in the same N2 gas atmosphere and then taken out from the furnace or cooled in a flow of an inert gas such as N2 even in the case that the oxide film is annealed in a vacuum atmosphere. With this method, however, it is not possible to sufficiently reduce the sheet resistance of the transparent conductive oxide film, and there are caused problems of increase in operation voltage of the light-emitting device, non-uniform light emission in the surface of the light-emitting layer, and so forth.